Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method of performing a device-to-device (D2D) operation using an exceptional resource by a user equipment (UE), and a UE using the method.
Related Art
In International Telecommunication Union Radio communication sector (ITU-R), a standardization task for International Mobile Telecommunication (IMT)-Advanced, that is, the next-generation mobile communication system since the third generation, is in progress. IMT-Advanced sets its goal to support Internet Protocol (IP)-based multimedia services at a data transfer rate of 1 Gbps in the stop and slow-speed moving state and of 100 Mbps in the fast-speed moving state.
For example, 3rd Generation Partnership Project (3GPP) is a system standard to satisfy the requirements of IMT-Advanced and is preparing for LTE-Advanced improved from Long Term Evolution (LTE) based on Orthogonal Frequency Division Multiple Access (OFDMA)/Single Carrier-Frequency Division Multiple Access (SC-FDMA) transmission schemes. LTE-Advanced is one of strong candidates for IMT-Advanced.
There is a growing interest in a Device-to-Device (D2D) technology in which devices perform direct communication. In particular, D2D has been in the spotlight as a communication technology for a public safety network. A commercial communication network is rapidly changing to LTE, but the current public safety network is basically based on the 2G technology in terms of a collision problem with existing communication standards and a cost. Such a technology gap and a need for improved services are leading to efforts to improve the public safety network.
The public safety network has higher service requirements (reliability and security) than the commercial communication network. In particular, if coverage of cellular communication is not affected or available, the public safety network also requires direct communication between devices, that is, D2D operation.
D2D operation may have various advantages in that it is communication between devices in proximity. For example, D2D UE has a high transfer rate and a low delay and may perform data communication. Furthermore, in D2D operation, traffic concentrated on a base station can be distributed. If D2D UE plays the role of a relay, it may also play the role of extending coverage of a base station.
Meanwhile, when a D2D operation is used for public safety, it may be very important to guarantee continuity of the D2D operation. For example, it is very important to guarantee that a user equipment (UE) continuously performs the D2D operation even when the UE is not configured with a resource for the D2D operation because of a physical layer problem with a network, a radio link failure, or the like. A resource that is set to be used by the UE in order to guarantee continuity of the D2D operation in an exceptional case mentioned as above is referred to as exceptional resources. The exceptional resources may be set in advance or may be set based on system information broadcasted by a cell.
However, a conventional exceptional resource is set in the assumption that a D2D operation is performed on a frequency as the same as a frequency on which a UE communicates with a serving cell. That is, the conventional exceptional resource is not set in consideration of the case where a frequency on which the UE communicates with the serving cell is different from a frequency on which the UE performs the D2D operation. Therefore, how to apply an exceptional resource, which is given in a frequency of the serving cell, to a frequency for performing the D2D operation is not yet defined and thus still remains ambiguous.